The discovery of the accelerating expansion of the Universe, thought to be driven by a mysterious form of ‘dark energy’ constituting most of the Universe, has further revived the interest in testing Einstein’s theory of General Relativity (GR). Frame-dragging in the gravitational field generated by a rotating body or by a current of mass-energy is one of the most fascinating phenomena predicted by GR. The recently launched LARES (Laser RElativity Satellite) space mission is aimed at improving of about an order of magnitude the accuracy of the previous frame-dragging measurements by the LAGEOS and LAGEOS 2 satellites, using GRACE-derived Earth gravity determinations. After some years of orbital analysis of LARES, LAGEOS and LAGEOS 2 satellite laser ranging data, we should reach a few percent uncertainty in testing frame-dragging.

However, at the very foundation of Einstein’s theory is the geodesic motion of a small, structureless ‘test-particle’. Depending on the physical context, a star, planet or satellite can behave very nearly like a test-particle, so geodesic motion is used to calculate the advance of the perihelion of a planet’s orbit and the dynamics of a binary pulsar system and of an Earth-orbiting satellite. Verifying geodesic motion is thus a test of paramount importance to GR and other theories of fundamental physics. On the basis of the first few months of satellite laser-ranging observations of the LARES satellite, its orbit shows the best agreement of any satellite with the test-particle motion predicted by GR. That is, after modelling its known non-gravitational perturbations, the orbit of LARES shows the smallest deviations from geodesic motion of any artificial satellite: its residual mean acceleration away from geodesic motion is less than 0.5 x 10-12 m/s2. LARES-type satellites and accurate satellite laser ranging measurements can thus be used for further tests of gravitational and fundamental physics.